What do I fill it with?

A “dry pack” deck mortar is used to fill the bench. Do not use Thin-Set mortar! The mix ratio of the described mortar is 4 parts sand to 1 part cement- you may also use a pre-bagged mortar mix which is available at many of the tile locations that stock Better-Bench products, additionally, you will find this mix at lumber yards, hardware stores, Home Depot, Lowes, etc., under the common name of; mason mix, brick mortar, or deck mud. You will be adding just enough water to the dry mix to allow you to “chop” ( a garden hoe works best!) the mixture to consistency that allows you to pick up a handful and shape it into a ball without getting it all over your hands. Be sure to blend the mixture to the point that all dry powder pockets have been eliminated.

Then what?
Fill the entire bench with the mortar, and pack it tight as you go with a wood float, block of wood or even a steel trowel. Make sure the material is in all corners completely. Allow the mortar to build up about a 1/4 to 3/8 of an inch along the back wall(s) of the unit and screed it flush to the front lip to provide adequate drainage of the horizontal surface (this is not necessary in “dry” areas). Then add a small amount of water to the remaining mix to make it a little bit stickier. You will then apply this mix to the front face and screed it flush with the top and bottom edge of the channel. Allow the unit to set sufficiently prior to beginning your tile process.